1. Field of the Invention
An embodiment of the invention relates to a lighting device. In particular, an embodiment of the invention relates to a lighting device utilizing electroluminescence.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements utilizing electroluminescence. A basic structure of a light-emitting element utilizing electroluminescence is that in which a layer including a light-emitting substance is sandwiched between a pair of electrodes. By applying voltage between the electrodes of the light-emitting element, light from the light-emitting substance can be obtained.
A light-emitting element utilizing electroluminescence includes, in its category, a light-emitting element using a light-emitting substance of an organic compound and that using a light-emitting substance of an inorganic compound. In particular, application of the light-emitting element using a light-emitting substance of an organic compound to a planar light source is expected because the light-emitting element is easily enlarged due to its manufacturing method.
As examples of the applications of a planar light source, a display device, a backlight of a display device, a lighting device, and the like are given. Since light extraction efficiency (luminance) is emphasized in any of these applications, various techniques have been proposed so far to improve light extraction efficiency.
One of these techniques is a method in which a three-dimensional miniaturized structure is formed over a glass substrate and the total reflection component at a surface is reduced, so that light extraction efficiency is improved (for example, see Patent Document 1).